1. Field of the Invention
The present invention is directed to a stabilized, non-racemic form of a cyanohydrin ester.
2. Description of the Prior Art
Certain esters of substituted cyclopropanecarboxylic acids and substituted phenylacetic acids are a useful class of pesticides known as "pyrethroids". These pyrethroids, especially the more recent synthetic ones, are of considerable interest because of their high insecticidal activity, quick knockdown activity, low persistence as toxic residues and their low mammalian toxicity.
The pyrethroids can exist in the form of optical isomers, cis-trans isomers and other kinds of geometric isomers depending on the particular substituent groups. These various isomers may have different pesticidal toxicities and/or knockdown potency. Thus, one may prefer to resolve (racemic) mixtures of isomers to recover a more pesticidally active isomer or isomer mixture or one may prepare a more active isomer or isomer mixture directly.
For example, in the case of certain cyanohydrin esters containing at least one asymmetric carbon atom in each of the acid and alcohol moieties, the individual diastereoisomers containing the (+)-R-stereoisomer configuration in the alcohol moiety are significantly less active than the individual diastereoisomers containing the (-)-S-stereoisomer configuration. Thus, it is desirable to prepare the more pesticidally active stereoisomers or non-racemic mixtures enriched in the more pesticidally active stereoisomer. An example of such a mixture is an enantiomer pair containing the more pesticidally active stereoisomer configuration. Direct synthesis or mixture enrichment with respect to a more pesticidally active non-racemic form of cyanohydrin esters is desirable and now possible by base-catalyzed epimerization (as hereinafter described in this application and in U.S. Pat. Nos. 4,133,826 and 4,151,195). Moreover, it is desirable to prevent reverse epimerization of the non-racemic ester back toward the starting racemic or diastereoisomer mixture.